The present invention relates to an autofocusing system for a camera which is adapted for use in forming an image of an object on a light receiving element of an amount-of-light integration type by means of a luminous flux incident through a lens and obtaining a defocus amount of the with respect to a object by integrating the amount of light.
So-called TTL (Through-The-Lens) type focus detection systems (wherein the detection of the defocus amount depends on the state of the image formed with luminous flux transmitted through a photographic lens) is widely employed, as an automatic focusing system in cameras, such as single-lens reflex cameras, which require a true in-focus condition.
In this case, a delivery-amount-of-lens conversion factor, Kval, is used to convert the defocus amount into AF (Auto-Focus) motor drive pulse numbers.
Kval in this specification is defined as a delivery amount of lens per unit movement of an image surface. The term Kval may also be defined as a movement of the image surface per unit delivery-amount of lens; in the case of the latter, however, the size relation therebetween is reversed in the following description.
Since the Kval assumes different values based on the focal length of the lens in the case of a zoom lens, it is stored as variable data in a ROM (Read Only Memory) that is provided in the lens unit. It is noted that a lens system having a variable focal length can be either a zoom lens system or a variable-focal lens system. Accordingly, although the following explanation will be given only with respect to the zoom lens system, it should be noted that the present invention is also similarly applicable to the variable-focal lens systems.
According to the aforementioned definition, the longer the focal length, the smaller the Kval value becomes, and vice versa, as represented by a curve shown by a broken line in FIG. 11. As the focal length of a lens is not directly detectable, the zoom ring position is shown, in FIG. 11, as a variable corresponding to the focal length.
Conventionally, Kval is obtained from a zoom code detected through brushes that are in slidable contact with a code plate supported by the zoom ring and a stationary ring of a lens barrel, whereby it is obtained as intermittent data providing one value between the zoom codes, as shown by a continuous line in FIG. 12. This is also applicable to a case where the focal length is detected as shown in FIG. 12. In order to distinguish between the actual focal length and what is detected from the zoom code plate, the latter is shown in FIG. 12 as a detected focal length.
If, however, the data stored in the lens ROM is addressed according to the zoom code detected from the zoom code plate, Kval tends to become roughly controlled. Therefore, accurate AF control may not be performed.
Furthermore, when the focusing lens stands still, the lens is only necessary to start driving it by obtaining the drive amount from the detected defocus amount and the aforementioned Kval.
In case the defocus amount is large, as shown in FIG. 13, the linearity of the Kval will not be ensured. Since Kval is determined over a linear range, the lens will stop at point B and may not be in focus, as shown by a dotted line in FIG. 13, if the defocus amount is detected at a point A to excute the AF control.
The defocus amount has to accordingly be detected in such a zone that Kval linearly varies to obtain an accurate drive amount. In other words, it becomes necessary to carry out CCD-integration and, when the defocus amount is large, to carry out the CCD-integration during the AF driving operation.
The amount to drive a lens is proportional to the AF-motor-drive pulse numbers and, provided the lens operates with a uniform motion, the drive-pulse numbers are also proportional to time. In FIG. 14, the number of pulses from a stop point up to the commencement of the CCD-integration is designated as P1; the number of pulses up to the termination of the integration is designated as P2; the number of pluses up to the termination of the conversion into the drive-pulse numbers from the defocus amount detected from the CCD output is designated as P3; and the number of pulses up to a target in-focus position is designated as P4.
With a simple calculation, the number of pulses obtained through the aforementioned calculation corresponds to the midpoint between the timing at which the integration is started (P1) and the timing at which it is terminated (P2), i.e. to the defocus amount when the lens is located at the position corresponding to the number of pulses (P1+P2)/2. However, the lens is actually located slightly closer by a number of pulses .alpha. to the target than the midpoint. As a result, an overlapping part equivalent to the number of pulses .alpha. may be included in the drive amount according to a simple calculation.
The number of drive pluses is therefore determined by subtracting the overlapping part .alpha. from the number of pulses up to the target, the number of which is obtained from the simple calculation when the drive pulse is computed during the AF driving operation. The calculation of the drive pulse during the AF driving operation is defined as "overlapping integration" in this specification.
If, however, zooming is conducted during the overlapping integration (in the case of a zooming lens), an accurate AF control cannot be effected because Kval changes by the zooming of the lens.